Function deriveFlightCategory

deriveFlightCategory(
    visibilitySm: number | undefined,
    isLessThan: boolean,
    sky: SkyCondition,
    isCavok: boolean,
): FlightCategory | undefined
Derives the flight category (VFR, MVFR, IFR, LIFR) from visibility and ceiling conditions.

The ceiling is defined as the lowest broken (BKN) or overcast (OVC) cloud layer, or vertical visibility if the sky is obscured. The flight category is determined by the most restrictive of ceiling or visibility.

Category Ceiling Visibility

VFR > 3,000 ft > 5 SM

MVFR 1,000 - 3,000 ft 3 - 5 SM

IFR 500 - 999 ft 1 - < 3 SM

LIFR < 500 ft < 1 SM
```
import { deriveFlightCategory } from '@squawk/weather';

const category = deriveFlightCategory(10, false, { layers: [{ coverage: 'FEW', altitudeFtAgl: 25000 }] }, false);
// Returns 'VFR'
```
Parameters
- visibilitySm: number | undefined
  Prevailing visibility in statute miles, or undefined if unknown.
- isLessThan: boolean
  True when visibility is reported as less than the stated value.
- sky: SkyCondition
  Sky condition with cloud layers and/or vertical visibility.
- isCavok: boolean
  True when CAVOK is reported (implies VFR conditions).
Returns FlightCategory | undefined
The derived flight category, or undefined if insufficient data to determine.
- Defined in packages/weather/src/flight-category.ts:31